In general, clutches have two objectives: to transmit full torque when closed and to transmit no torque when open. The prior art teaches wet clutches with clutch drag from frictional contact between rotating components of the clutch such as a piston, pressure plates, and friction plates, when open. The clutch drag reduces the efficiency of a component, such as a transmission, including the clutch. The prior art teaches reducing such clutch drag by separating a piston, pressure plates, and friction plates by greater respective amounts. However, the prior art also teaches that greater amounts of hydraulic fluid are needed to displace the separated elements, increasing the time needed to close the clutch. For example, if the piston is located further from the pressure plates, more fluid is needed to displace the piston the further distance. The above options create a conflict between fuel economy (reduced by clutch drag) and acceptable driveability (response time for closing the clutch).
There are other problems associated with reducing clutch drag by increasing gaps in the clutch. When using a fixed lever or hydraulic ratio to engage a clutch, it becomes difficult to find the touch point (when torque begins to be transmitted) when attempting to close the increased gaps quickly. The difficulty in finding the touch point may result in either slow shift times or rough shifts. Furthermore, clutch drag is often exacerbated in the case of wet clutches in order to control tolerances in fabrication. That is, because a smaller lift-off gap is easier to control, gaps between components in the clutch are even further reduced. Variable ratio mechanisms have been proposed; however, such mechanisms have problems with complexity, friction, or are difficult to apply to rotating clutches.